# In order of appearance "in" or "on" the chain

I want to say

array that stores positions of nucleotides in order of appearance in (or is it on?) the S chain.

I know it sounds too specific as a genetics kind of question, but is just for a comment, let's say it was a chain of letters.
I don't know if I'm saying it right, but I'm trying to say that a given array (something to do with programming, -unimportant, but is similar to a shopping cart list, if you like-) will store the position of a nucleotide when it first appears in the chain S.
For example, let's say we've got the chain, where every A, C, G & T is a nucleotide, then

``````S = "CAGCCTA"
``````

will store in my list [1,6,0,3,4,2,5], because at position 1 in S (first nucleotide -C- is at position 0) we've found an A, and I'm storing nucleotides in order of appearance as explained above, first the As (the second A's at position 6, hence the 6 after the 1), then the Cs, etc...
I googled first and found a few references about lists, for instance on the list as correct in many contexts (like, your name is on the list, and in the list as correct when talking about lists in programming, but I'm talking about a chain here, a chain of nucleotides in this case, which is sort of a normal list of nucleotides, maybe.

• I believe either is grammatical, but you might want to consult genetics texts to see which is more usual in this context. Usage of "in" vs "on" has some general rules, but a lot is based on convention or habit. Oct 2 '19 at 12:38
• I see your point @StuartF, it is quite specific, but let's say it was a chain of any letter, would it be on the chain or in the chain? ill edit my question. Oct 2 '19 at 12:53
• If you think of each base pair or nucleotide as a link in the chain, then I would think of it as a link "in the chain". Oct 2 '19 at 13:35